transformer: add support for instrumenting the V compiler with `-d tr…

…ace_transformer`

CONTRIBUTING.md

@@ -285,6 +285,7 @@ a copy of the compiler rather than replacing it with `v self`.
 | `trace_scanner`                   | Prints details about the recognized tokens. *Very* verbose. Use `./vnew -no-builtin -check-syntax file.v` later.    |
 | `trace_parser`                    | Prints details about parsed statements and expressions. Very verbose. Use it for panics in the parser.              |
 | `trace_checker`                   | Prints details about the statements being checked. Very verbose. Use it for panics in the checker.                  |
+| `trace_transformer`               | Prints details about the statements being transformed. Very verbose. Use it for panics in the transformer stage.    |
 |                                   |                                                                                                                     |
 | `trace_gen`                       | Prints all the strings written to the generated C file. Very verbose.                                               |
 | `trace_cgen_stmt`                 | Prints details about the statements that are being processed by cgen.                                               |

vlib/v/transformer/transformer.v

@@ -174,6 +174,10 @@ pub fn (mut t Transformer) check_safe_array(mut node ast.IndexExpr) {
 }
 
 pub fn (mut t Transformer) stmt(mut node ast.Stmt) ast.Stmt {
+	$if trace_transformer ? {
+		ntype := typeof(*node).replace('v.ast.', '')
+		eprintln('transformer: ${t.file.path:-50} | pos: ${node.pos.line_str():-39} | node: ${ntype:12} | ${node}')
+	}
 	match mut node {
 		ast.EmptyStmt {}
 		ast.NodeError {}